1. Technical Field
The present invention relates generally to a combustion monitoring apparatus for an internal combustion engine, and more particularly to a combustion monitoring apparatus designed to distinguish an abnormal combustion condition from a normal combustion condition using a combustion ion current resulting from motion of ions between plug electrodes of a spark plug which are produced by combustion of an internal combustion engine.
2. Background of Related Art
An internal combustion engine for an automotive vehicle usually uses a spark plug to establish an arc between plug electrodes for igniting an air-fuel mixture injected into a combustion chamber. Upon the ignition of the mixture, combustion begins and spreads in all directions from the spark plug within the combustion chamber to produce an engine power. Combustion conditions are constantly changed according to traveling conditions of the vehicle. A conventional engine control system, thus, monitors generation of abnormal combustion in order to control ignition timing of a spark plug and an air/fuel ratio for maintaining a proper combustion condition at all times. For example, abnormal combustion called combustion knock results in loss of power and in excessive increase in combustion chamber pressure, causing physical damage to the engine. In order to avoid this abnormal combustion, a knocking sensor is in general use which is mounted in a combustion chamber to detect generation of an abnormal vibration caused by combustion knock for retarding ignition timing. The knocking sensor, however, may detect vibrations produced by factors other than the combustion knock. In order to avoid this drawback, Japanese Patent First Publication No. 58-7536 teaches a knocking detecting apparatus which applies a dc voltage to plug electrodes of a spark plug to detect an ion current resulting from motion of ions between the plug electrodes for determining the strength of combustion based on the fact that a large number of ions are produced upon generation of detonation causing the combustion knock.
Usually, in order to improve fuel consumption or control exhaust emissions, an air/fuel ratio is shifted to a lean side or an exhaust gas is recirculated to purge nitrogen oxides contained in the exhaust gas. These, however, may induce unstable combustion such as a misfire and combustion knock. It is, thus, essential to control combustion conditions properly, and the combustion conditions need to be detected with high accuracy.
In the above prior art knocking detecting apparatus, it is difficult to detect only the ion current because of influence of charging and discharging currents flowing through the plug electrodes functioning as capacitive elements. Moreover, a dc high voltage applied to the plug electrodes causes combustion ions to be absorbed by the plug electrodes, so that the ion current becomes unstable, thereby decreasing the accuracy in detecting the abnormal combustion conditions.